1. Technical Field
The present invention relates to the polishing of semiconductor wafers of the type from which chips for integrated circuits and the like are made. More specifically in a chemical mechanical process (CMP) a semiconductor wafer is held by a tooling head and is polished by contact with an abrasive material in a controlled chemically active environment.
2. Discussion of the Related Art
As part of the manufacturing process of semiconductor devices, semiconductor wafers are polished by CMP. The uniform removal of material from and the planarity of patterned and un-patterned wafers is critical to wafer process yield. Generally, the wafer to be polished is mounted on a tooling head which holds the wafer using a combination of vacuum suction or other means to contact the rear side of the wafer and a retaining lip or ring around the edge of the wafer to keep the wafer centered on the tooling head. The front side of the wafer, the side to be polished, is then contacted with an abrasive material such as an abrasive pad or abrasive strip. The abrasive pad or strip may have free abrasive fluid sprayed on it, may have abrasive particles affixed to it, or may have abrasive particles sprinkled on it.
The ideal wafer polishing process can be described by Preston's equation: R=K.sub.p *P*V, where R is the removal rate; Kp is a function of consumables (abrasive pad roughness and elasticity, surface chemistry and abrasion effects, and contact area); P is the applied pressure between the wafer and the abrasive pad; and V is the relative velocity between the wafer and the abrasive pad. As a result, the ideal CMP process should have constant cutting velocity over the entire wafer surface, constant pressure between the abrasive pad and wafer, and constant abrasive pad roughness, elasticity, area and abrasion effects. In addition, control over the temperature and Ph is critical and the direction of the relative pad/wafer velocity should be randomly distributed over the entire wafer surface.
Most current CMP machines fail to produce constant velocity distribution over the entire wafer surface which is necessary for uniform material removal and good flatness. One common type of wafer polishing apparatus is the CMP model 372M made by Westech Systems Inc. As shown in simplified form in FIG. 1, a wafer 100 is held by a tooling head 102 which rotates about the axis of the wafer. A large circular abrasive pad 104 is rotated while contacting the rotating wafer and tooling head. The rotating wafer contacts the larger rotating abrasive pad in an area away from the center of the abrasive pad. Thus in the Westech apparatus, the relative motion between the wafer and the abrasive pad has two components: one due to the rotating wafer and another due to the rotating abrasive pad.
A number of disadvantages result from the relative motion between the wafer and the abrasive pad in the Westech apparatus. According to Preston's equation the rate at which material is removed from a given point on the wafer is directly proportional to the relative velocity between that point and the abrasive pad. In the Westech apparatus, different points on the wafer see different relative abrasive pad velocities, and therefore have different removal rates. The non-uniform relative velocities result from the fact that the velocity near the center of a rotating circle is less than the velocity near the outside of the circle. For example, while the center of the rotating wafer sees a constant velocity which is related solely to the rotating abrasive pad, the outer points of the wafer see velocities which are a combination of the rotating tooling head and the rotating abrasive pad.
Not only does the Westech apparatus result in non-uniform velocities at different points of the wafer at any one time, the velocities of points away from the center tend to fluctuate significantly over time. Constant velocities are preferable to fluctuating velocities since the removal rate and other factors necessary in obtaining a smooth finish are much easier to control. For example, with the Westech system, points away from the center of the wafer see alternating high and low velocities. During a period of low velocity, the abrasive material may pit or scratch the surface of the wafer and result in a non-smooth surface.
Another related apparatus is a polishing machine for polishing semiconductor wafers containing magnetic read-write heads, disclosed in U.S. Pat. No. 5,335,453 to Baldy et al. With this machine, a semiconductor wafer is held by a support head which is moved in a circular translatory motion by an eccentric arm. The wafer is polished by contacting an abrasive strip which is advanced in one direction. The relative motion between the wafer and the abrasive strip is a combination of the circular motion of the wafer and the linear motion of the advancing abrasive strip. The resulting relative motion is that of a circle precessing in a straight line. Note that "precessing" used herein refers to the movement in a plane of an axis which is perpendicular to the plane and about which a circular translation occurs.
While the precessing circle polishing pattern that this apparatus provides should provide more uniform velocities such that different points on the wafer see similar velocities at any given time, the velocities are still not constant. Assuming the rotation of the eccentric arm is held to a constant angular speed, the precessing circle relative motion results in fluctuating velocities. When the wafer is rotating away from the precessing direction the net relative velocity is lower, and when the wafer is rotating with precessing direction the net relative velocity is higher.
Moreover, the apparatus has the disadvantage of not being able to provide alternative polishing patterns. Since the support head is mounted on a rotating eccentric arm, the wafer can only be polished by moving in a circle. Polishing patterns other than circular are desired for a number of reasons.
One such reason is to provide more uniform wear of the abrasive pad. Non-uniform wear of the abrasive pad results in a non-uniform removal rate of wafer material since more heavily worn sections of the abrasive pad remove material at a lower rate. Non-uniform wear also results in less efficient use of the abrasive pad itself, since the pad must be changed more often or advanced at a faster rate in order to avoid using portions of the pad which wear out first.
A relative polishing motion of a circle precessing in a straight line results in the center of the abrasive strip having more wear than the outside edges. This is because the wafer spends more time in the center of the pad than at the outer edges. The more time the wafer spends on certain sections of the abrasive strip, the more the abrasive strip wears out.
Another disadvantage of circular patterns is uncontrolled spinning of the wafer inside the tooling head. The forces from the polishing motion in a continuous circular motion cause the wafer to tend to spin or rotate in one direction with respect to the tooling head.
An additional disadvantage of only providing a circular pattern is that certain topologies on the wafer surface may be less well suited for circular polishing patterns. The pre-polishing topology of the surface of the wafer may be patterned, due to processing. Each surface topology is optimally planarized by a certain polishing pattern, which may not always be a circular one. Thus, providing the ability to custom design polishing patterns is desirable so that polishing different surface topologies may be optimized.
Another disadvantage with prior systems is that it is difficult or impossible to polish selected regions of the wafer using a specific portion of an abrasive pad. For example, if a zone of aggressive abrasive is present on the abrasive pad, it is difficult to selectively use that zone to increase the removal rate on certain parts of the wafer, if the only motion provided by the polishing system is that of a fixed radius circle.
Another disadvantage with prior systems is related to the fact that a wafer moving in a particular direction will have a higher removal rate on the leading edge and sides of the wafer. Prior art systems providing only certain polishing patterns cannot be easily made to control the removal rates at the edges due to this effect. Circular motion generally provides a constantly variable cutting direction and therefore will cause removal rates from all the edges. However, prior art systems cannot be programmed so that certain areas are selectively polished by spending more time traveling in one direction than another.
While Preston's equation, explained above, describes the ideal process, it does not address some factors related to a manner in which the wafer is presented to the polishing media and chemistry. Many CMP machines currently available yield wafers bearing anomalies in planarity. These anomalies are often related to factors that cannot be described by Preston's equation. For example, it has been found with many available CMP machines, that the removal rates of material are higher near the edges of the wafer. The shape of the platen (which holds the wafer against the polishing media) and the relationship between the platen and the retaining ring (which keeps the wafer centered on the platen) are crucial to the final wafer planarity.
One attempt at addressing some of the problems not described by Preston's equation is described in U.S. Pat. No. 5,205,082 to Shendon et al. Disclosed is an attempt to control the relationship between the platen, ring and pad by tying the platen and the ring together through a flexible diaphragm. However, by allowing the ring to float with respect to the platen, the ring can be upset by changes in abrasive pad flatness, roughness, and friction. When the ring is disturbed, the pressure on the periphery of the wafer increases. This can contribute to poor planarity because of more pronounced oxide removal rates near the wafer edges.
Another type of tooling head, described in U.S. Pat. No. 4,954,142 to Carr et al., uses a retaining ring with constant pressure at all points on the ring. The pressure on the ring is not adjustable during polishing, in that different springs must be used for any variation in conditions, such as changes in the abrasive pad surface.
Changing springs requires disassembly of the retention ring from the head. Thus there is a need for a tooling head having a retaining ring in which the pressure between the ring and the abrasive pad is easily adjustable, and less prone to changes in the abrasive pad surface.
Other problems with tooling heads include undesirable chattering or vibration during certain polishing patterns. Chattering or vibration is caused by allowing a certain degree of backlash, or play between the tooling head and its supporting member in directions parallel to the wafer being polished.